The present invention relates to a method for determining the state of health of a battery comprising a least one rechargeable cell.
An electrochemical cell or rechargeable cell (both terms being equivalent) is a device for producing electricity in which chemical energy is converted into electrical energy. The chemical energy is constituted by electrochemically active compounds deposited on at least one face of electrodes disposed in the cell. The electrical energy is produced by electrochemical reactions while the cell is discharging. The electrodes, which are placed in a container, are electrically connected to battery output terminals providing electrical continuity between the electrodes and a load with which the cell is associated.
The battery is designed to supply electrical energy to an external application and the charging circuit is generally provided to which the battery can be connected in order to charge the cells. To increase the power delivered, practice is to associate several sealed rechargeable cells in order to constitute a battery. The battery can include one or several parallel branches of series-connected cells. Battery charging and discharge management can then be organized and controlled in order to balance the charging and discharge of the various cells one with respect to the others. Management system, including measurement sensors and an electronic control circuit of greater or lesser sophistication depending on the applications may be associated with the battery.
The State of Charge (SOC) and State of Health (SOH) constitute useful information for an electronic battery management system, for optimizing use and lifetime. Typically, the state of charge SOC is determined as being the amount of energy available in the battery; it can be calculated by integrating charging/discharge current with respect to time. The state of health SOH of the battery makes it possible to determine ageing of the battery between a new condition and a deteriorated condition; it can be calculated as the ratio between battery impedance at a given point in time and battery impedance in its new condition, at the same temperature.
Determining the state of health of a battery as directly as impedance ratios is however not always reliable and can lead to a battery which is still in good condition being replaced or, on the contrary, to a lack of anticipating a failure, prejudicing the application for which the battery is intended.
A first way of determining the state of health of a battery consists in performing regular maintenance checks in order to determine internal resistance and/or capacitance of the battery, while disconnected from the external application or the charger. This means however that the battery is not available during these maintenance operations, which can be prejudicial to certain applications. Further, there is no evaluation of SOH between two maintenance operations.
Another way of determining state of health of a battery consists in monitoring values for temperature, and optionally voltage and optionally current exchanged, in order to determine a predictive value for SOH on the basis of tests previously performed in the laboratory. Any departure of battery behavior compared to nominal behavior will however not be detected. Such a method of determination is not reliable.
U.S. Pat. No. 7,202,632 proposes measuring battery impedance when certain criteria (of temperature and variation in current) are met, and then calculating SOH from this measurement as being the ratio of the calculated impedance to the initial impedance with application of weightings. The weightings defined in this patent correspond to a filter in the time domain. Measurements are however not performed continuously and it can happen that the SOH does not get evaluated over a fairly long period if the measurement conditions are not met.
U.S. Pat. No. 7,009,401 proposes measuring battery impedance at varying frequencies and with certain measurement conditions (for example, discharge current kept constant during measurement). This method does consequently also not provide continuous measurement of battery impedance regardless of operating conditions.
U.S. Pat. No. 7,324,902 discloses a method for determining the stage of charge and state of health of a battery by defining a complex model of the battery and performing a recursive determination starting from this model and functional relations between initial parameters. This method does not introduce weighting depending on battery state.
U.S. Pat. No. 7,072,871 proposes measuring a certain number of battery parameters and using fuzzy logic to calculate battery state of health. This method does not take account of the state of the battery, but solely the type of battery employed.
U.S. Pat. No. 6,534,954 discloses the use of a Kalman filter for determining state of charge. US-A-2006/0111854 discloses a method for battery internal parameter determination based on the use of a Kalman filter. The Kalman algorithm is an efficient way of determining states of a dynamic system using incomplete or noisy measurements. Nevertheless, the Kalman algorithm alone does not have sufficient stability to furnish a stable value for battery state of health. In effect, by definition, the Kalman algorithm increases variation on the parameter (for example impedance) it is monitoring when the parameter departs from the model, which can be the case when the battery is at a low SOC, or experiencing significant temperature variations. Under such conditions, the Kalman algorithm cannot but amplify SOH determination error.
The article “Li-ion battery SOC estimation method based on the reduced order Kalman filtering” by J. Lee et al, Journal of Power Sources, 174, 2007 (9-15) proposes determining battery impedance by a Kalman algorithm with a weighting that is dependent upon SOC. The values determined by the algorithm are rejected in regions of high or low SOC or when heavy currents are passing. Such weighting renders the Kalman algorithm more robust but does not make it possible to continuously monitor state of health of the battery regardless of battery operating conditions.
EP-A-1 688 754 discloses a method for measuring SOC. However, this method does not account on reliability of the measures.